Snell has recently changed its rating system. Prior to, I believe, 2008, a Snell rated helmet was "harder" due to the test platform being subjected to two impacts on the same area of the helmet. Naturally, to pass the test, the helmet had to me slightly more dense.
Any DOT rated helmet will work, Snell is a matter of preference, but not absolutely necessary for safety (and maybe not wanted in a smaller-headed rider, as more G-forces are transferred to the head....).
As for choosing a helmet, find one that fits and is comfortable...no use wearing something for hundreds of hours that bugs you.
I have an Arai XD that I paid about $600 for, including tax, in 2005. I've replaced the shield once, and have really loved being able to take out the liner for cleaning; without this feature the helmet would just be plain gross by now. The back of the lining has been sewed where it began to tear, but the bottom line is that the helmet fits like a glove, and a forget that I'm wearing it, and can wear it for 10+ hour days for weeks on end.
An excellent helmet that has been worth every penny.
To the contrary, I have an 'el cheapo $79 AFX that is my "spare". It's a torture device. Lining is scratchy, non-removeable, fits (my head), with tons of gaps and is almost too loud with wind noise.
Best bet...go try some helmets on. Find a comfortable one in your price range...and wear it!
Wearing full gear (All the Gear, All the Time) is one of the best ways you can put some odds in your favor on a bike.
-H-